Function when

Start a when, then, otherwise expression.

Example

// Below we add a column with the value 1, where column "foo" > 2 and the value -1 where it isn't.
> df = pl.DataFrame({"foo": [1, 3, 4], "bar": [3, 4, 0]})
> df.withColumn(pl.when(pl.col("foo").gt(2)).then(pl.lit(1)).otherwise(pl.lit(-1)))
shape: (3, 3)
┌─────┬─────┬─────────┐
│ foo ┆ bar ┆ literal │
│ --- ┆ --- ┆ ---     │
│ i64 ┆ i64 ┆ i32     │
╞═════╪═════╪═════════╡
│ 1   ┆ 3   ┆ -1      │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┤
│ 3   ┆ 4   ┆ 1       │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┤
│ 4   ┆ 0   ┆ 1       │
└─────┴─────┴─────────┘

// Or with multiple `when, thens` chained:
> df.with_column(
...     pl.when(pl.col("foo").gt(2))
...     .then(1)
...     .when(pl.col("bar").gt(2))
...     .then(4)
...     .otherwise(-1)
... )
shape: (3, 3)
┌─────┬─────┬─────────┐
│ foo ┆ bar ┆ literal │
│ --- ┆ --- ┆ ---     │
│ i64 ┆ i64 ┆ i32     │
╞═════╪═════╪═════════╡
│ 1   ┆ 3   ┆ 4       │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┤
│ 3   ┆ 4   ┆ 1       │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┤
│ 4   ┆ 0   ┆ 1       │
└─────┴─────┴─────────┘
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